The thin film magnetic recording disk in a conventional drive assembly typically consists of a substrate, an underlayer consisting of a thin film of chromium (Cr) or a Cr alloy, a cobalt-based ferromagnetic alloy deposited on the underlayer, and a protective overcoat over the ferromagnetic layer. The word "magnetic" will be used herein as a synonym for "ferromagnetic." A variety of disk substrates such as NiP-coated AlMg, glass, glass ceramic, glassy carbon, etc., have been used. The microstructural parameters of the magnetic layer, i.e., crystallographic preferred orientation (PO), grain size and magnetic exchange decoupling between the grains, play key roles in controlling the recording characteristics of the disk. The Cr underlayer is mainly used to control such microstructural parameters as the PO and grain size of the cobalt-based magnetic alloy.
The PO of the various materials forming the layers on the disk, as discussed below, is not necessarily an exclusive orientation which may be found in the material, but is merely the most prominent orientation. When the Cr underlayer is sputter deposited at a sufficiently elevated temperature on a NiP-coated AlMg substrate a [100] PO is usually formed. This PO promotes the epitaxial growth of [1120] PO of the hexagonal close-packed (hcp) cobalt (Co) alloy, and thereby improves the in-plane magnetic performance of the disk. The [1120] PO refers to a film of hexagonal structure whose (1120) planes are predominantly parallel to the surface of the film. (Likewise the [1010] PO refers to a film of hexagonal structure whose (1010) planes are predominantly parallel to the surface of the film).
Nucleation and growth of Cr or Cr alloy underlayers on glass and most non-metallic substrates differ significantly from NiP-coated AlMg substrates (AlMg/NiP).
Magnetic films fabricated on glass substrates are often noisier than films on AlMg/NiP substrates under otherwise identical deposition conditions. It is for this reason that magnetic media on non-metallic substrates are structured differently than on AlMg/NiP substrates. "Glass" will be used herein for convenience to refer to the entire class of non-metallic substrates unless otherwise noted. Glass substrate disks, for example, benefit from the use of an initial layer called the seed layer. The seed layer is formed on the glass substrate beneath the underlayer in order to control nucleation and growth of the underlayer which in turn affects the magnetic layer. Various materials have been proposed for seed layers such as: Al, Cr, Ni.sub.3 P, Ta, C, W, FeAl and NiAl on non-metallic substrates. Laughlin, et al., have described use of an NiAl seed layer followed by a 2.5 nm thick Cr underlayer and a CoCrPt magnetic layer. The NiAl seed layer with the Cr underlayer was said to induce the [1010] texture in the magnetic layer. (See "The Control and Characterization of the Crystallographic Texture of Longitudinal Thin Film Recording Media," IEEE Trans. Magnetic. 32(5) September 1996, p.3632).
Various designs of thin film disks with laminated magnetic layers are known. The typical laminated magnetic layers are cobalt alloys separated by a thin layer of nonmagnetic material such as Cr or Cr alloy. The multiple magnetic layers in laminated disks are all typically composed of the same alloy.
Since longitudinal recording requires that the C-axis be sufficiently oriented in the plane of the substrate, the range of thin film structures which might otherwise be used is restricted. For example, CoPtCrB (quaternary boron or QB) alloys with high chromium content have a tendency to orient with the C-axis vertical to the plane of the substrate. Some QB alloys have certain advantages as described in U.S. Pat. No. 5,523,173, so it is useful to find ways to overcome the PO problem.
The '173 patent describes special sputtering conditions which are useful in depositing QB on an AlMg/NiP substrate to pull the C-axis more strongly into the plane of the substrate. The desired orientation of the QB in the '173 patent is [1120] PO in which the C-axis is sufficiently in the plane of the substrate for longitudinal recording. One of the conditions in the '173 method is 300 volts of negative bias on the substrate during sputtering of the Cr underlayer. Unfortunately, the '173 negative bias technique is impractical when glass substrates are used due to the electrically insulating nature of glass. Therefore, use of QB on glass substrates to obtain the advantages thereof in an efficient manufacturing process, requires a novel approach.